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Ministry of Agriculture

Apiculture Factsheet #506

Blue Orchard Mason Bee,
Osmia lignaria

A number of species of Orchard Mason Bees are native to North
America. They occur in different climatic environments but are
particularly well adapted in the northern ranges of blooming fruit
trees. The common Blue Orchard Mason Bee, Osmia lignaria is
found in the B.C.’s southern Interior and coastal areas. It goes
under various names including Blue Orchard Bee, Orchard Mason Bee,
Orchard Bee, and Osmia Bee.

The Orchard Mason Bee belongs to the family Megachilidae which is
comprised of a large number of solitary pollinators. Included are
leaf cutter bees such as the Alfalfa Leafcutter Bee, Megachile
roduntata that is widely cultivated in North America for alfalfa
pollination.

The most important species in the genus Osmia in north America
are the subspecies found west of the Rockies Osmia lignaria
propingua, and Osmia lignaria lignaria widely distributed
east of the Rockies. Other native subspecies have limited
distribution ranges. Osmia bees have also proven to be effective in
other parts of the world. In Japan, the species Osmia cornifrons
has been used as the primary pollinator in apple cultivation.

Physical Appearance

The Orchard Mason Bee appears black but is actually dark metallic
green/blue in color. The female is approximately 14 mm in length,
robust in appearance resembling a black fly. The male is smaller and
more slender, and about 11-12 mm in length. Males are characterized
by their long antennae and a tuft of light colored hair in the front
of the head. At rest, the bee has its wings flush with its body.
Osmia bees are effective pollinators because of their pubescence
or hairiness. This enables them to carry pollen grains from flower
to flower, causing pollination to take place.

Mason Bee

Foraging Behavior

Orchard Mason bees occur in woodlands and forest edges. They
appear in early spring when the first bushes and trees bloom.
Cherry, Pear and Apple are particularly attractive but other nectar
and pollen sources include Quince, Laburnum spp and blueberry. Osmia
bees are fast flyers and display a high bloom visitation rate. Its
high activity, even under poor weather conditions, make this insect
pollinator particularly attractive for early blooming crops.

Orchard Mason Bees are shy and fly away when disturbed. Even at
their nests, female bees will not display defensive behavior even
though, they are capable of stinging. Similarly to honeybees,
Orchard Mason Bees gather nectar in their ‘honey sac’ while
foraging. The nectar is used as energy source and to provision the
tubular nest.

Unlike honeybees, Orchard Mason Bees do not have specially
modified hind legs called corbicula to store and carry
pollen. Instead, pollen is packed underneath rows of stiff hairs
called scopa under the abdomen.

Nest Sites and Breeding Behavior

Osmia bees are solitary insects and complete their lifecycle on
their own. Most species are gregarious in that they nest close
together. This behavior offers several advantages such as lower
predation pressures, increased mating opportunity, and optimized
genetic variability through cross breeding. It is this gregarious
behavior that has offered the opportunity to “domesticate” the
Orchard Mason Bee.

Mated females will use existing holes in wood for a nest. Holes
with 7 - 8 mm diameter holes are favored. A mud plug is placed at
the end of the tubular nest and then she will place up to 20 loads
of nectar and pollen at the end of the tube. When sufficient food
has been deposited an egg is laid and the cell is sealed with a thin
mud plug. The whole process is repeated for each egg and cell she
creates until the tube is filled close to its entrance. Often the
last cell is left empty to discourage predators. The tube is then
closed with a thick mud plug at the entrance. Some wasp species also
use tubular nests but their end plugs are often smooth and while the
plug of the Mason bee is always rough.

The female Mason bee lives for about one month in the spring and
she can produce one or two eggs a day. One tubular nest contain 7 -
11 cells where those laid first, in the back of the tube will
develop into females while the few cells nearest to the entrance
will be males.

Brood Development

A few days after the egg has been laid, the larva will hatch and
will start feeding on the nectar and pollen reserves. The larva
grows very rapidly and after 10 - 14 days most food reserves have
been consumed. The larva will spin a cocoon and pupate. Later in the
summer, the pupa will develop into an adult and will stay in the
cell throughout the winter.

In early spring when the first warm days occur, male Osmia Bees
will first emerge. They chew their way through the mud plug with
their strong mandibles. The males will stay near the tubular nests
and wait for females to emerge. As soon as females appear, the males
will attempt to mate. There is fierce competition between males and
sometimes, a female is covered by a number of struggling males.

Developing your own Osmia Bee Population

Osmia Bees occur in many parts of southern British Columbia.
Local populations are often limited by the availability of suitable
nesting sites and forage sources. Providing suitable nest sites will
attract Osmia Bees and allows for the establishment of a sizable
Osmia Bee population in an area.

Alternatively, there are various commercial sources available in
British Columbai where a “starter population” comprised of a few
tubes can be purchased and installed in the garden with a larger
number of empty tubes for nesting sites.

Nests can be made from blocks of wood, 2x4’s and 4x4’s, with
holes of 7 - 8 mm in diameter. The length of the hole is not
critical but should be 10 - 15 centimeters without opening at the
end. It is recommended to use pine or fir but not cedar since the
latter contains resins that repel insects. Alfalfa Leaf Cutter Bee
boards with holes of at least 6 mm can also be used.

Suitable nests can also be created with cardboard and paper
straws. Cardboard straws can be bundled together and wrapped in
weather resistant tarpaper or inserted into a large plastic tube.
Wooden blocks can also be used more efficiently by drilling larger
holes of 9 - 10 mm and insert paper sleeves which can be removed in
winter.

In October, place blocks or tubular nests in garden shed or
garage where there are no temperature extremes and predators. When
using paper sleeves, remove from the wooden blocks and place in
tray. The tray should be netted or screened and placed in
refrigerator or in a cool, dry place until spring. In mid-March,
nest blocks should be placed to receive morning sun. Bee activity
continues until June and blocks should be removed and placed in the
shed by mid July.

Mud Availability

As their name indicates, Osmia Bees need access to mud. If a
source is readily available near the nests, the females can be
spared a great deal of time and labor. A patch of soil can be kept
moist or a small bucket or tray can be filled with wetted soil.

Pollen Mites

Emerging male and female Osmia bees sometime have a brown colored
mottled covering on their thorax and abdomen. These are large
numbers of pollen mites that feed on the pollen that was left after
the bee larva entered its pupal stage. These pollen mites are not
believed to be injurious to the bees but could possibly limit food
availability to the bee larva in spring, and pose a physical
hindrance to the adult bee in flight.

Cardboard and paper inserts may offer an effective means to lower
the effects of the pollen mites. In winter, tubes can be carefully
cut lengthwise and the bees and nesting material can be dipped in a
cleansing solution for a short period. Unlike wooden blocks without
inserts, cardboard and paper inserts are not reused and can help to
prevent pollen mite buildup.

Orchard Pollination

The Osmia Bee has proven to be a very effective pollinator of
tree fruit orchards. Their high activity level and tendency to visit
different trees optimizes cross pollination. Effective crop
pollination can be attained with a much smaller number of Osmia Bees
as compared to an equal number of honeybees. Yet, monocultural
practices demand huge numbers of pollinating insects in blooming
crops.

For an apple orchard, it is recommended to provide 500 - 1000
filled holes per acre. Assuming an occupancy level of 1.5 females
per hole, this would provide for up to 1000 female bees. Females are
the primary pollinators as they are the sole nest builders. Males
also pollinate but their foraging is only done for nourishment.

Osmia Bees often disperse after emergence and seek nesting holes
in other areas. For this reason, population increase in a locality
may be slow and insufficient to meet the pollination requirements of
a crop or orchard. With proper management, a sizable population can
be established after several years.